In recent years, steels for welded structures have been required to have low yield strength and high uniform elongation in addition to high strength and high toughness from the viewpoint of earthquake-proof. For example, steels for line pipes used in quake zones which may possibly be deformed significantly are required to have low yield strength and high uniform elongation in some cases. In general, it is known that the yield strength and uniform elongation of steel can be reduced and increased, respectively, in such a manner that the metallographic microstructure of the steel is transformed into a microstructure in which a hard phase such as bainite or martensite is adequately dispersed in ferrite, which is a soft phase.
As for manufacturing methods capable of obtaining a microstructure in which a hard phase is adequately dispersed in a soft phase as described above, Patent Literature 1 discloses a heat treatment method in which quenching (Q′) from the two-phase (γ+α) temperature range of ferrite and austenite is performed between quenching (Q) and tempering (T).
As for methods in which the number of manufacturing steps is not increased, Patent Literature 2 discloses a method in which after rolling is finished at the Ar3 transformation temperature or higher, the start of accelerated cooling is delayed until the temperature of a steel material decreases to the Ar3 transformation temperature, at which ferrite is produced, or lower.
As for techniques for achieving low yield ratio without performing such heat treatment as disclosed in Patent Literature 1 or 2, Patent Literature 3 discloses a method in which low yield ratio is achieved in such a manner that after the rolling of a steel material is finished at the Ar3 transformation temperature or higher, the rate of accelerated cooling and the finishing cooling temperature are controlled such that a two-phase microstructure consisting of acicular ferrite and martensite is produced.
Furthermore, as for techniques for achieving low yield ratio and excellent welded heat affected zone toughness without significantly increasing the amount of an alloying element added to steel, Patent Literature 4 discloses a method in which a three-phase microstructure consisting of ferrite, bainite, and island martensite (M-A constituent) is produced in such a manner that Ti/N and/or the Ca—O—S balance is controlled.
Patent Literature 5 discloses a technique in which low yield ratio and high uniform elongation are achieved by the addition of an alloying element such as Cu, Ni, or Mo.
On the other hand, welded steel pipes, such as UOE steel pipes and electric welded pipes, used for line pipes are manufactured in such a manner that steel plates are cold-formed into pipes, abutting surfaces thereof are welded, and the outer surfaces of the pipes are usually subjected to coating such as polyethylene coating or powder epoxy coating from the viewpoint of corrosion resistance. Therefore, there is a problem in that the steel pipes have a yield ratio greater than the yield ratio of the steel plates because strain ageing is caused by working strain during pipe making and heating during coating and the yield stress is increased. In order to cope with such a problem, for example, Patent Literatures 6 and 7 each disclose a steel pipe which has excellent strain ageing resistance, low yield ratio, high strength, and high toughness and which contains fine precipitates of composite carbides containing Ti and Mo or fine precipitates of composite carbides containing two or more of Ti, Nb, and V and also disclose a method for manufacturing the steel pipe.